Position-measuring devices for measuring the angular position of a shaft are familiar from a multitude of publications. Such position-measuring devices are referred to as rotary encoders. Moreover, if a position-measuring device is designed such that, in addition to the angular position of the shaft, it is also possible to measure the number of revolutions executed by the shaft, this is referred to as a multiturn rotary encoder.
In principle, two design approaches are used for implementing the multiturn unit, e.g., the unit for determining the number of revolutions executed by the shaft, first of all, multiturn units based on gears, and secondly, multiturn units based on counters.
In the case of gear-based multiturn units, the input shaft actuates one or perhaps several gear stages which gear down the input speed. Given a gear stage having a reduction ratio of 16:1, for example, the output shaft of the gear stage rotates one time per 16 revolutions of the input shaft. The output shaft in turn drives a code carrier, whose angular position allows conclusions about the number of revolutions of the input shaft per unit time.
An example for a gear-based multiturn rotary encoder is described in German Published Patent Application 28 17 172. It relates to a multi-stage incremental shaft encoder having a first angular-increment code disk and a plurality of downstream angular-increment code disks.
European Published Patent Application No. 1 457 762 describes a device for measuring the position, the displacement, or the rotational angle of an object. It includes three measuring standards in the form of three successive code disks, which are coupled via a differential toothed gear. The code disks are scanned by a scanning unit radially covering the code tracks of all code disks.
Gear-based multiturn units are independent of a power supply per se, that is, they function when the rotary encoder is in the switched-off state, as well. That is quite simply because the code disks, driven by the gearing, rotate even without a power supply. After the rotary encoder has been switched on again, the number of revolutions executed by the input shaft may be determined by ascertaining the angular position of the code disks of the multiturn unit.
Counter-based multiturn units ascertain the number of revolutions executed by a shaft by counting the revolutions of a code carrier which is driven directly by the shaft, and therefore executes the same number of revolutions as the shaft to be measured. Located on the code carrier is a code which is scanned by a scanning unit. Based on the position signals ascertained by the scanning unit, counting signals are generated in counter electronics for a counter which counts—as a function of the direction of rotation—the number of complete revolutions of the code carrier, and therefore of the shaft. Without further measures, counter-based multiturn units are dependent on a power supply, that is, when the power supply of the rotary encoder is switched off, first of all, the value of the counter is lost, and secondly, revolutions of the shaft can no longer be counted. After being switched on again, the counter is reset.
In order to store the counter reading of the counter even when the main power supply is switched off, e.g., for example, when the machine in which the multiturn rotary encoder is operated is switched off, and in addition, to maintain the counting function, counter-based multiturn units are frequently equipped with a battery which takes over the energy supply for at least the multiturn unit of the rotary encoder upon loss of the main power supply.
For example, European Patent No. 1 462 771 describes a multiturn rotary encoder having a counter-based multiturn unit which is operated with a battery when in the switched-off state.
These and other measures which make it possible to upgrade a counter-based multiturn unit to function independently of a power supply are costly, and are therefore to be avoided. Even the use of a battery is problematic, since even when working with power-saving electrical circuits, sooner or later it is necessary to change the battery.
For safety-related reasons, it may be necessary to measure the number of revolutions of a shaft redundantly, for example, when a faulty measurement may lead to damage of a machine on which the multiturn rotary encoder is operated, or may even result in danger for the operating personnel of the machine.
Therefore, German Published Patent Application No. 10 2009 029 431 describes a multiturn rotary encoder which is equipped with two multiturn units that are independent of a power supply. A comparison of the measured values of the multiturn units makes it possible to check the performance reliability of the multiturn rotary encoder at all times. It is therefore suitable for use in safety-related applications.